[Elecraft] K3 if isolation of softrock lite used as panadapter

[Jerry Flanders]

Thanks, Jack

I just recently realized that many people assume that one sets the 
softrock center freq to the center of the IF passband. This would 
require MUCH isolation to keep the LO under control. This is NOT how 
I would recommend it be used. I would set the softrock's oscillator 
below the IF passband, outside the area passed by the roofing filter.

Since all I ever use in my panadaptor with either my 756PRO3 or 
SDR-1000 or sofrtock 40 is about +/- 12 KHz anyway, I will be quite 
happy setting the LO to 12 KHz below the IF center, and let the SR 
panadaptor display the K3 tuned freq +/- 12 KHz.

With this design, isolation problems may be simplified. Also, there 
is no need for a premium soundcard.

I hope that, with the LO well outside the IF passband, we will need 
considerably less isolation. I actually have a premium soundcard 
available, so I can move the LO even farther away if that helps. But 
I won't know for sure until mine comes in and I warm up the soldering iron.

Jerry W4UK

At 06:14 AM 2/12/2008, you wrote:
>Jerry:
>
>You might first wish to measure the reverse isolation of a MMIC 
>before you make that statement. While they are great devices, 
>isolation is not their strong suit.
>
>The Gali74, which I use for a variety of purposes, has an isolation 
>of around 28 dB at the K2's IF frequency. The gain is 25 dB, so if 
>you pad the output for unity  gain, you are around 53 dB. With a 
>Softrock's typical -40 dBm LO leakage, that strikes me a bit on the 
>marginal side.
>
>
>Jack K8ZOA
>
>
>
>Jerry Flanders wrote:
>>I assume a simple MMIC amp plus a simple resistive attenuator 
>>circuit will provide enough isolation and also provide for matching 
>>levels up/down as may be necessary. At least, that will be my first 
>>cut at the problem unless somebody else comes up with a better plan 
>>before my K3 arrives.
>>
>>At down east microwave ( http://www.downeastmicrowave.com/cat-frame.htm ),
>>see the 
>><http://literature.agilent.com/litweb/pdf/5967-6159E.pdf>INA10386 
>>(DC-2GHz, 24dBG, 3.5dB NF +14dbm 1dB Gain compression ) for only 
>>$5. Very easy to use something like this, I think.
>>
>>Depending on the signal level of the K3 IF output, we might get 
>>enough isolation from  a resistive attenuator alone and still have 
>>a usable signal level on the softrock panadaptor.
>>
>>I would much prefer a plug and play manufactured solution if it 
>>takes more than an attenuator, but not an expensive one.
>>
>>Jerry W4UK
>>
>>
>>At 07:29 PM 2/11/2008, stephen pearce wrote:
>>>A month or so ago one of the Elecraft engineers pointed out that
>>>if the softrock lite rx is connected directly to the if port on the k3
>>>there was a significant issue with coupling of rf from the softrock
>>>board into the
>>>rx if of the k# with consequent degradation in rx performance ...
>>>
>>>Has anyone produced a design for a suitable isolation amplifier/
>>>coupling stage which will
>>>prevent this and still allow the softrock to be used (in conjunction
>>>with a pc) as a panadapter?
>>>
>>>Thanks
>>>
>>>
>>>--
>>>Stephen Pearce
>>>zl1any
>>>Whangarei
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>>
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